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Allow limited inbound ICMP to Nexus, add ICMP type/code filters to firewall rules #8194

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@FelixMcFelix FelixMcFelix commented May 21, 2025

This PR adds more detail to firewall protocol filters. Rather than being specified purely as a string->enum mapping, each protocol filter is now a standard tagged enum like many other types in the Nexus API. Here, this allows us to express more control over ICMP traffic. For instance:

  • all ICMP ([{"type": "icmp"}]),
  • all ICMP echo ([{"type": "icmp", "value": { icmp_type: 0 }}, {"type": "icmp", "value": { icmp_type: 8 }}])
  • a subset of ICMP destination unreachable ({"type": "icmp", "value": { icmp_type: 0, code: "0-4" }}).

Building on this, Nexus now has a firewall entry to permit the receipt of e.g., Destination Unreachable messages. I've added a new system endpoint to control whether this is enabled or disabled.

As part of this, I've converted the CRDB representation from a fixed ENUM[] to a STRING(32)[]. There are a couple of reasons for this:

Should close #7998.

@FelixMcFelix FelixMcFelix force-pushed the felixmcfelix/icmp-for-nexus branch from 5594fc0 to 38bd2ff Compare May 21, 2025 12:46
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FelixMcFelix commented May 23, 2025

I've setup these bits on dublin this morning. After performing the schema update, everything seems happily functional, and posting true/false to the new system endpoint correctly enables/disables the ICMP allowance as expected. Console is broken when we go to the firewall rules tab, but that is to be expected given the OpenAPI changes.


On the actual MTU dscovery front, it looks as though both this PR and main are doing the correct thing?

If from the web console I perform a 'Reload (Override Cache)' we can kick off large enough TCP transfers to run into LSO. When we trace this through:

Dublin

BRM23230018 # dtrace -n 'xde_mc_tx:entry{ this->m = (mblk_t *)arg1; this->mss = this->m->b_datap->db_struioun.cksum.pad } xde_mc_tx:entry/this->m->b_datap->db_struioun.cksum.flags && this-
>mss != 0xBADD && this->mss/{printf("packet with MSS %d", this->m->b_datap->db_struioun.cksum.pad);}'
dtrace: description 'xde_mc_tx:entry' matched 2 probes
CPU     ID                    FUNCTION:NAME
111   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
  1   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
 69   5301                  xde_mc_tx:entry packet with MSS 1368
^C

BRM23230018 # dtrace -n 't4_eth_tx:entry{ this->m = (mblk_t *)arg1; this->mss = this->m->b_datap->db_struioun.cksum.pad } t4_eth_tx:entry/this->m->b_datap->db_struioun.cksum.flags && this-
>mss != 0xBADD && this->mss/{printf("packet with MSS %d", this->m->b_datap->db_struioun.cksum.pad);}'
dtrace: description 't4_eth_tx:entry' matched 2 probes
CPU     ID                    FUNCTION:NAME
  9   6360                  t4_eth_tx:entry packet with MSS 8928
 34   6360                  t4_eth_tx:entry packet with MSS 8928
117   6360                  t4_eth_tx:entry packet with MSS 8928
120   6360                  t4_eth_tx:entry packet with MSS 8928
110   6360                  t4_eth_tx:entry packet with MSS 8928
120   6360                  t4_eth_tx:entry packet with MSS 8928
 86   6360                  t4_eth_tx:entry packet with MSS 8928
 86   6360                  t4_eth_tx:entry packet with MSS 8928
 77   6360                  t4_eth_tx:entry packet with MSS 8928
 78   6360                  t4_eth_tx:entry packet with MSS 8928
 87   6360                  t4_eth_tx:entry packet with MSS 1368
  8   6360                  t4_eth_tx:entry packet with MSS 8928
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 78   6360                  t4_eth_tx:entry packet with MSS 8928
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 43   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 43   6360                  t4_eth_tx:entry packet with MSS 1368
 43   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 78   6360                  t4_eth_tx:entry packet with MSS 8928
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 69   6360                  t4_eth_tx:entry packet with MSS 1368
 43   6360                  t4_eth_tx:entry packet with MSS 1368
 43   6360                  t4_eth_tx:entry packet with MSS 1368
^C

The latter query is also catching underlay traffic, hence the 8928B entries. An MSS of 1368 matches what I'd maybe expect for a reduced MTU of 1402 bytes (resp. 1448 for a 1500B MTU). Do we have any hits on the new rules?

BRM23230018 # opteadm dump-layer -p opte0 firewall
Port opte0 - Layer firewall
======================================================================
Inbound Flows
----------------------------------------------------------------------
PROTO  SRC IP        SPORT  DST IP      DPORT  HITS  ACTION
TCP    172.20.17.94  50056  172.30.2.6  443    0     no-op
TCP    172.20.17.94  50062  172.30.2.6  443    0     no-op

Outbound Flows
----------------------------------------------------------------------
PROTO  SRC IP      SPORT  DST IP        DPORT  HITS  ACTION
TCP    172.30.2.6  443    172.20.17.94  50056  1     no-op
TCP    172.30.2.6  443    172.20.17.94  50062  1     no-op

Inbound Rules
----------------------------------------------------------------------
ID   PRI    HITS  PREDICATES                               ACTION
126  65534  2     inner.ip.proto=TCP                       "Stateful Allow"
                  inner.ulp.dst=80,=443

125  65534  0     inner.icmp.type=time exceeded            "Stateful Allow"
124  65534  0     inner.icmp.type=destination unreachable  "Stateful Allow"
                  inner.icmp.code=4

DEF  --     427   --                                       "deny"

Outbound Rules
----------------------------------------------------------------------
ID   PRI  HITS  PREDICATES  ACTION
DEF  --   2     --          "stateful allow"

Apparently not, although it's hard to tell given that the reconcile task is completely reinstating the rules periodically so we're losing those counters. Note that all the deny entries here are from my own ping https://recovery.sys.dublin.eng.oxide.computer command.

Dogfood

BRM44220011 # dtrace -n 'xde_mc_tx:entry{ this->m = (mblk_t *)arg1; this->mss = this->m->b_datap->db_struioun.cksum.pad } xde_mc_tx:entry/this->m->b_datap->db_struioun.cksum.flags && this-
>mss != 0xBADD && this->mss/{printf("packet with MSS %d", this->m->b_datap->db_struioun.cksum.pad);}'
dtrace: description 'xde_mc_tx:entry' matched 2 probes
CPU     ID                    FUNCTION:NAME
 41   4868                  xde_mc_tx:entry packet with MSS 1448
  9   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
127   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 68   4868                  xde_mc_tx:entry packet with MSS 1368
 83   4868                  xde_mc_tx:entry packet with MSS 1368
 83   4868                  xde_mc_tx:entry packet with MSS 1368
 83   4868                  xde_mc_tx:entry packet with MSS 1368
 83   4868                  xde_mc_tx:entry packet with MSS 1368
^C

BRM44220011 # dtrace -n 't4_eth_tx:entry{ this->m = (mblk_t *)arg1; this->mss = this->m->b_datap->db_struioun.cksum.pad } t4_eth_tx:entry/this->m->b_datap->db_struioun.cksum.flags && this->mss != 0xBADD && this->mss/{printf("packet with MSS %d", this->m->b_datap->db_struioun.cksum.pad);}'
dtrace: description 't4_eth_tx:entry' matched 2 probes
CPU     ID                    FUNCTION:NAME
 18   5927                  t4_eth_tx:entry packet with MSS 8928
 52   5927                  t4_eth_tx:entry packet with MSS 8928
 52   5927                  t4_eth_tx:entry packet with MSS 1448
113   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 76   5927                  t4_eth_tx:entry packet with MSS 8928
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1448
 44   5927                  t4_eth_tx:entry packet with MSS 1448
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 18   5927                  t4_eth_tx:entry packet with MSS 1368
 18   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
120   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
^C
 76   5927                  t4_eth_tx:entry packet with MSS 8928
 95   5927                  t4_eth_tx:entry packet with MSS 1368
 78   5927                  t4_eth_tx:entry packet with MSS 1368
 43   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 43   5927                  t4_eth_tx:entry packet with MSS 1368
 43   5927                  t4_eth_tx:entry packet with MSS 1368
 60   5927                  t4_eth_tx:entry packet with MSS 1368
 43   5927                  t4_eth_tx:entry packet with MSS 1368
 60   5927                  t4_eth_tx:entry packet with MSS 1368
 60   5927                  t4_eth_tx:entry packet with MSS 1368
  1   5927                  t4_eth_tx:entry packet with MSS 1368
113   5927                  t4_eth_tx:entry packet with MSS 1368
  1   5927                  t4_eth_tx:entry packet with MSS 1368
 69   5927                  t4_eth_tx:entry packet with MSS 1368
 86   5927                  t4_eth_tx:entry packet with MSS 1368
 52   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 42   5927                  t4_eth_tx:entry packet with MSS 1368
 42   5927                  t4_eth_tx:entry packet with MSS 1368
 42   5927                  t4_eth_tx:entry packet with MSS 1368
103   5927                  t4_eth_tx:entry packet with MSS 1368
 19   5927                  t4_eth_tx:entry packet with MSS 1368
 19   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368
 68   5927                  t4_eth_tx:entry packet with MSS 1368

There's a mix in there, but I assume we have something on the lab-side hitting the API periodically, since I see those 1448 entries passively. But dogfood appears to be discovering and using the reduced MTU just fine.


Fixing oxidecomputer/opte#748 may have been sufficient to get this working, and that PR has been pulled into main already. As a fun bonus, due to this work I can now run traceroute from the Nexus zone, so we might want to expand the scope of the allow rule across external DNS too. The NTP zones can't really benefit since they're behind a SNAT.

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I think this is good for review, I've pointed out some open design points I'm unsure on.

@@ -136,7 +136,7 @@ z_swadm () {

# only set this if you want to override the version of opte/xde installed by the
# install_opte.sh script
OPTE_COMMIT=""
OPTE_COMMIT="792ec8a3816ba8c7c4268f65cd19dbd946a9027d"
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Noting that this needs to be removed as part of merging.

Comment on lines +159 to +166
// Port assignments are incompatible with non
// TCP/UDP protocols.
if matches!(
proto,
ProtoFilter::Tcp | ProtoFilter::Udp
) {
filters.set_ports(ports.clone());
}
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I'm surprised we haven't run into this one yet -- if someone were to install a rule on [TCP, ICMP] x [port 80, port 443] this would previously be installed as the two rules:

  • proto = TCP && (port = 80 || port = 443), [valid]
  • proto = ICMP && (port = 80 || port = 443). [invalid]

In OPTE, a port match will always return false unless it is applied to TCP/UDP traffic.

Comment on lines +95 to +97
targets: vec![VpcFirewallRuleTarget::Subnet(
super::vpc_subnet::NEXUS_VPC_SUBNET.name().clone(),
)],
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Do we want to include any more services here?

Comment on lines +100 to +112
protocols: Some(vec![
VpcFirewallRuleProtocol::Icmp(Some(VpcFirewallIcmpFilter {
// Type 3 -- Destination Unreachable
icmp_type: 3,
// Code 4 -- Fragmentation needed
code: Some(4.into()),
})),
VpcFirewallRuleProtocol::Icmp(Some(VpcFirewallIcmpFilter {
// Type 11 -- Time Exceeded
icmp_type: 11,
code: None,
})),
]),
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This gives us PTMUD (although it looks as though the TCP stack can figure it out in the absence of Fragmentation Needed messages?), and the ability to use traceroute from Nexus/DNS. Is the latter needed? Are there any other ICMP families which would be crucial?

Comment on lines 1980 to 1982
async fn networking_inbound_icmp_view(
rqctx: RequestContext<Self::Context>,
) -> Result<HttpResponseOk<bool>, HttpError>;
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This works on a bool today to have a simple toggle. Would we rather have a Vec<VpcFirewallIcmpFilter> (or similar) here?

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In general, I think we prefer to avoid JSON bodies that are just a single primitive type not wrapped in a response object as it's caused issues with client coe generation the past; see the final paragraph in this comment from @david-crespo:

A related but distinct issue is that we generally want API request and response bodies to be JSON objects rather than strings. We've run into issues with client generation when that has not been the case. A string is valid JSON, but I think this would be the only endpoint that returns a string. One reason to prefer objects is that objects can be extended by adding another key without changing the basic shape of the thing. If these unlikely to change their shape, consistency with other endpoints is probably a stronger argument.

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Yeah, I think { "enabled": boolean } would be better. In addition to the above excellent points, it's a nice extra bit of unavoidable documentation about what the boolean represents.

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That sounds good to me, it should be a simple enough change. I wasn't aware of the impact on clients here.

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Should be done in 5c7caed.

@FelixMcFelix FelixMcFelix marked this pull request as ready for review May 27, 2025 10:39
@FelixMcFelix FelixMcFelix added this to the 16 milestone May 27, 2025
Comment on lines +86 to +87
String::from_sql(bytes)?
.parse::<external::VpcFirewallRuleProtocol>()?,
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i had a vague memory of there being some way to do this by turning the bytes into an &str with str::from_utf8 or something, so that we don't make a whole String just to parse it and throw it away...but i wasn't able to find prior art for that elsewhere in Nexus, so i dunno if it's worth the effort.

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I had a quick look there and I think our options are limited here -- I couldn't find any other obvious examples in nexus itself, either.

Frustratingly, PgValue does have the right method for us, but this is not part of any trait shared by Backend::RawValues. We also don't have any FromSql<Text, _> for &[u8]/*const [u8]. The next best thing is, seemingly, deserializing to a *const str. Kinda gross, and quoth the docs:

We have to return a raw pointer instead of a reference with a lifetime due to the structure of FromSql

Less than ideal. It would work great in a blanket impl for all FromStr types (where we could justify the unsafe), except I think the orphan rule would prevent us from trivially doing so. I suspect there's something clever we could do, but maybe upending everyone's FromSql<sql_types::Text, DB> impls is a bridge too far in this PR? I kinda want to see if it can be done in a followup.

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Nexus ICMP paranoia breaks Path MTU Discovery
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